During the past several decades, the use of capacitors formed using valve metal powders has grown exponentially. This increase is mainly due to a large growth in the use of solid tantalum capacitors. Solid tantalum capacitor use has increased due to their high reliability, high capacitance per unit volume, and wide variety of surface-mount configurations available.
Also contributing to the popularity of solid tantalum capacitors is the continuing decrease in cost per unit capacitance for these devices. The reduction in the cost per unit capacitance is, in part, the result of the increasing economy of scale. As ever greater numbers of the devices are manufactured ever more quickly, the fixed costs per capacitor are reduced, thereby fueling the market for these devices. Another very important factor in the continuation of cost reduction for solid tantalum capacitors is the availability of finer, higher surface area tantalum powders. The use of tantalum powders, having greater surface area per unit weight, allows the use of less tantalum powder per device, thereby facilitating a savings in the "contained tantalum" component of device cost.
Unfortunately, as tantalum capacitor powders having higher surface areas per unit weight have come into use, several disadvantages of these finer (i.e. smaller particle size) powders have become apparent. Finer powders exhibit less-than-ideal flow characteristics during the anode pressing process. The generally slower and less even flow characteristics of finer tantalum powders results in less uniform anode weights unless slower machine speeds are employed; this, in turn, makes the anode fabrication process less efficient as fewer parts are produced per unit of time.
The finest particles present in higher surface area capacitor powders tend to become airborne readily during processing on anode presses, necessitating expensive explosion-resistant high air flow rate exhaust systems to prevent injury to workers and to reduce the fire/explosion hazard from airborne dust. The dust from high surface area capacitor powders has also proven to be highly abrasive in contact with the dies, punches, sliding, and rotary bearing surfaces of anode pressing equipment. The presence of the fine dust from high surface area tantalum powders requires the use of more precise punch and die tolerances, cemented carbide tooling in place of hardened steel, and frequent bearing replacement, all of which add to the cost of capacitor anode fabrication with these powders.
The simple expedient of employing a powdered binder/lubricant material, such as ethylene diamine bis d-stearamide (sold under the tradename of "Acrawax", by the Lonza Corporation) in mechanical mixture with the higher surface area capacitor powders imparts lubricity to these powders, minimizing anode press repairs due to wear, but results in very little improvement in flow properties or fine dust generation.
The coating of fine capacitor powders with binder/lubricant via tumbling the powders in a solution of the binder/lubricant (such as a solution of the binder, stearic acid, dissolved in one or more chlorinated solvents and/or acetone), followed by dynamic vacuum-drying of the binder-coated capacitor powder in a Patterson-Kelly V-shell type blender results in a reduction of fine powder dust generation, as well as improved pressing equipment lubrication, but does not address powder flow considerations.
An additional problem is observed with high surface area capacitor powders, such as tantalum having a surface area above about 0.3 square meter per gram, which is that traditional binder/lubricant materials become increasingly more difficult to remove completely. Tantalum powders having a surface area of 0.4-0.5 square meter per gram, mixed with 1% to 5% stearic acid or ACRAWAX (with or without the use of a solvent) and pressed into 0.1 gram anode pellets are frequently found to contain 300 to 400 ppm carbon after a thermal binder removal step in vacuum and 150 to 200+ ppm carbon following the vacuum sintering step used to produce the finished anode bodies prior to electroprocessing (anodizing, counter-electrode fabrication, and encapsulation). The level of carbon remaining in the anode bodies after vacuum-sintering is proportionally higher with progressively finer capacitor powders and larger anode size.
The presence of carbon on the valve metal surfaces within the interstices of the anodes after vacuum sintering leads to the production of anodic oxide having flaws or high electrical leakage regions. These flaws are thought to be due to the presence of spots of tantalum carbide on tantalum anode surfaces; the tantalum carbide is thought to give rise to holes or thin spots in the tantalum oxide film which conduct electricity under the application of voltage (this leakage current mechanism is discussed in Young's 1961 book, Anodic Oxide Films, in the chapters dealing with tantalum). Whatever the mechanism, the correlation between elevated levels of carbon in anodes after vacuum sintering and high finished device leakage currents has been empirically established for many years.